29.08.2017 |

World’s soils have lost 133bn tons of carbon, new research shows

Agriculture has removed 133bn tons of carbon from the top 2 meters of soil (Photo: CC0)

The world’s soils have lost 133 billion tons of carbon from the top 2 meters of soil since the beginning of agriculture around 12,000 years ago, new research suggests. And the rate of soil organic carbon loss has increased dramatically over the past 200 years since the start of the industrial revolution, according to the study published in the journal Proceedings of the National Academy of Sciences on August 21. The research team found that agriculture has historically released almost as much carbon into the atmosphere as deforestation. “The spread of agriculture has created a large carbon debt in soils,” said lead author Dr. Jonathan Sanderman of the Woods Hole Research Center, a US-based climate change think tank. However, “it has been difficult to estimate the size and spatial distribution of soil organic carbon loss from land use and land cover change.” The authors write that conclusions of previous studies on global soil carbon losses have varied widely, with estimates ranging from 40 billion to 500 billion tons of carbon. The authors now generated a new estimate using a machine learning-based model, a global compilation of soil carbon data, and the History Database of the Global Environment (HYDE) land use data.

Projection of this model onto a world without farming indicated a global carbon debt due to agriculture of 133 billion tons for the top two meters of soil. These soil organic carbon losses are on par with estimates of carbon lost from living vegetation primarily due to deforestation. “Historically, I think we’ve underestimated the amount of emissions from soils due to land use change,” Sanderman told the Washington Post. The study also showed that “grazing” and “cropland” contributed nearly equally to the loss of soil organic carbon. Even though there were higher percent losses on cropland, slightly higher total losses were found from grazing land since more than twice the surface of land is grazed. According to the study, the rate and extent of decline in soil organic carbon stocks varies greatly across the globe, due to differences in soil properties, climate, type of land-use conversion, and, importantly, the specific management implementation of a given form of land use. The authors also highlight that loss of soil organic carbon under agricultural land use is not universal; modest gains are seen when soil of naturally low fertility is improved and the previous constraint on plant growth is alleviated.

The authors say their results can provide a basis for national and international policies to target soil organic carbon restoration efforts. “Our maps indicate hotspots of soil carbon loss, often associated with major cropping regions and degraded grazing lands, suggesting that there are identifiable regions that should be targets for soil carbon restoration efforts,” they write. “The large soil carbon debt can be thought of as the maximum potential for soils to remove carbon from the atmosphere and act as a natural climate solution. Even realizing only a fraction of this potential would be an important climate mitigation strategy,” Sanderman said. The researchers point out that sustainable land management practices which help to put carbon back into the ground, such as efficient crop rotation, cover crops and changes in tillage practices, could make a great difference. “Modifying large-scale agricultural practices to restore some of these lost soil carbon stocks might be a valuable strategy in our efforts to dampen climate change,” co-author Dr Thomas Crowther from the Yale Climate and Energy Institute told CarbonBrief. “If regenerative agriculture can restore some of the carbon that we have lost, then it might be a really valuable tool in our fight against climate change.” (ab)

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